Pin diodes with over-current protection

ABSTRACT

A system includes a pixel including a diffusion layer in contact with an absorption layer. A transparent conductive oxide (TCO) is electrically connected to the diffusion layer. An overflow contact is in electrical communication with the TCO. The overflow contact can be spaced apart laterally from the diffusion layer. The pixel can be one of a plurality of similar pixels arranged in a grid pattern, wherein each pixel has a respective overflow contact, forming an overflow contact grid offset from the grid pattern.

BACKGROUND 1. Field

The present disclosure relates to diodes, and more particularly tophotodiodes such as used in pixels for imaging.

2. Description of Related Art

The lower the dark current of photodiodes in an imaging device, thebetter is the image quality. Similarly, the higher the sensitivity ofphotodiodes in imaging devices, the better is the image quality. At highlight levels, the excess photons generate high current, which may damagea detector and/or put more stress on the read-out integrated circuit(ROIC).

The conventional techniques have been considered satisfactory for theirintended purpose. However, there is an ever present need for improvedsystems and methods for photodiodes used in imaging devices. Thisdisclosure provides a solution for this need.

SUMMARY

A system includes a pixel including a diffusion layer in contact with anabsorption layer. A transparent conductive oxide (TCO) is electricallyconnected to the diffusion layer. An overflow contact is in electricalcommunication with the TCO.

The overflow contact can be spaced apart laterally from the diffusionlayer. The pixel can be one of a plurality of similar pixels arranged ina grid pattern, wherein each pixel has a respective overflow contact,forming an overflow contact grid offset from the grid pattern. Theoverflow contact can be metallic. A cap layer can be deposited on theabsorption layer opposite a substrate. The cap layer can include InP. ASiNx layer can be deposited over the cap layer. The TCO can be depositedon the SiNx layer, wherein the TCO conforms around the SiNx layer tocontact the diffusion layer. At least one additional SiNx layer can bedeposited over the TCO, wherein the overflow contact extends through theat least one additional SiNx layer. An anti-reflective layer can bedeposited on the substrate opposite the absorption layer. A contactmetal can be electrically connected to the diffusion layer, configuredto electrically connect the diffusion layer to a read-out integratedcircuit (ROIC). The overflow contact can be in electrical communicationwith the ROIC.

The overflow contact can be electrically isolated from the contactmetal, and wherein the TCO is electrically connected to a current Sinkof the ROIC. The TCO can include multiple layers of ZnO, TiO2 and/orIndium Tin Oxide (ITO). The absorption layer can include InGaAs, e.g.,wherein the pixel is sensitive to illumination in infrared wavelengths.It is also contemplated that the absorption layer can include Si, e.g.,wherein the pixel is sensitive to illumination in visible lightwavelengths.

A method includes forming a pixel array including a plurality of pixels,each pixel including a diffusion layer in contact with an absorptionlayer. The method includes forming a respective overflow contactelectrically connected with each respective pixel of the plurality ofpixels, wherein the overflow contacts follow an overflow contact grid.

The method can include depositing a SiNx layer on a cap layer that isdeposited on the absorption layer and depositing a transparentconductive oxide (TCO) on the SiNx layer, wherein the overflow contactsare electrically connected to the TCO. The method can include depositingat least one additional SiNx layer on the TCO.

These and other features of the systems and methods of the subjectdisclosure will become more readily apparent to those skilled in the artfrom the following detailed description of the preferred embodimentstaken in conjunction with the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

So that those skilled in the art to which the subject disclosureappertains will readily understand how to make and use the devices andmethods of the subject disclosure without undue experimentation,preferred embodiments thereof will be described in detail herein belowwith reference to certain figures, wherein:

FIG. 1 is a schematic cross-sectional elevation view of an embodiment ofa system constructed in accordance with the present disclosure, showingthe overflow contact; and

FIG. 2 is a schematic plan view of the system of FIG. 1, showing thegrid pattern for a plurality of pixels.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Reference will now be made to the drawings wherein like referencenumerals identify similar structural features or aspects of the subjectdisclosure. For purposes of explanation and illustration, and notlimitation, a partial view of an embodiment of a system in accordancewith the disclosure is shown in FIG. 1 and is designated generally byreference character 100. Other embodiments of systems in accordance withthe disclosure, or aspects thereof, are provided in FIG. 2, as will bedescribed. The systems and methods described herein can be used toreduce excess current, increase sensitivity, and reduce stress onread-out integrated circuits (ROICs) in imaging devices.

A system 100 includes a pixel 102 including a diffusion layer 104 incontact with an absorption layer 106. A transparent conductive oxide(TCO) 108 is electrically connected to the diffusion layer 104. Anoverflow contact 110 is in electrical communication with the TCO 108.The overflow contact 110 is spaced apart laterally, i.e. in thehorizontal direction as oriented in FIG. 1, from the diffusion layer104.

With reference now to FIG. 2, the pixel 102 of FIG. 1 is one of aplurality of similar pixels 102 arranged in a grid pattern 112,indicated with the phantom lines in FIG. 2, wherein each pixel 102 has arespective overflow contact 110, forming an overflow contact grid 114offset from the grid pattern 112.

With reference again to FIG. 1 a cap layer 116 can be deposited on theabsorption layer 106 opposite a substrate 118. The cap layer 116 caninclude InP. A SiNx layer 120 can be deposited over the cap layer 116.The TCO 108 can be deposited on the SiNx layer 120, wherein the TCO 108conforms around the SiNx layer 120 to contact the diffusion layer 104(i.e., the TCO wraps downwardly around the SiNx layer 120 to contact thediffusion layer 104 as oriented in FIG. 1. Two additional SiNx layers122, 124 are deposited over the TCO 108. The overflow contact 110extends through the additional SiNx layers 122, 124. An anti-reflectivelayer 126 can optionally be deposited on the substrate 118 opposite theabsorption layer 106. A contact metal 128 is electrically connected tothe diffusion layer 104, which electrically connects the diffusion layer104 to a read-out integrated circuit (ROIC) 130. The overflow contact110 is electrically connected to the ROIC 130.

The TCO 108 normally acts as an insulator, but when current reaches apredetermined maximum level, the resistance barrier of the TCO 108breaks and excess current can flow through the TCO 108 to a commoncurrent Sink 132 on the ROIC 130. The film resistivity of the TCO 108can be designed based on detector working conditions, which can beimplemented by adjusting the doping level of the TCO 108. The overflowcontact 110 can be metallic and is electrically isolated from thecontact metal 128. The TCO 108 can include multiple layers of ZnO, TiO2and/or Indium Tin Oxide (ITO).

The absorption layer 106 can include InGaAs, e.g., wherein the pixel 102is sensitive to illumination in infrared wavelengths. It is alsocontemplated that the absorption layer 106 can include Si, e.g., whereinthe pixel 102 is sensitive to illumination in visible light wavelengths.Those skilled in the art will readily appreciate that any other suitablematerial can be used to provide sensitivity in any other suitablewavelengths.

A method includes forming a pixel array (e.g. the array of pixels 102 ofthe square tiled grid pattern 112 in FIG. 2) including a plurality ofpixels. Each pixel includes a diffusion layer, e.g. diffusion layer 104,in contact with an absorption layer, e.g. absorption layer 106. Themethod includes forming a respective overflow contact, e.g. overflowcontact 110, electrically connected with each respective pixel of theplurality of pixels, wherein the overflow contacts follow an overflowcontact grid, e.g. the overflow contact grid 114 of FIG. 2.

The method can include depositing a SiNx layer, e.g. SiNx layer 120, ona cap layer, e.g. cap layer 116, that is deposited on the absorptionlayer and depositing a transparent conductive oxide (TCO), e.g. TCO 108,on the SiNx layer, wherein the overflow contacts are electricallyconnected to the TCO. The method can include depositing at least oneadditional SiNx layer, e.g. the additional SiNx layers 122, 124, on theTCO. The diffusion layer 104 is the P portion of a PIN diode, theabsorption layer 106 is the I portion of the PIN diode, and thesubstrate 118 is the N portion of the PIN diode.

The methods and systems of the present disclosure, as described aboveand shown in the drawings, provide for reduced excess current, increasedsensitivity, and reduced stress on read-out integrated circuits (ROICs)in imaging devices. This can improve image quality and can reduce ROICdesign requirements and signal processing complications relative totraditional configurations. While the apparatus and methods of thesubject disclosure have been shown and described with reference topreferred embodiments, those skilled in the art will readily appreciatethat changes and/or modifications may be made thereto without departingfrom the scope of the subject disclosure.

What is claimed is:
 1. A system comprising: a pixel including adiffusion layer in contact with an absorption layer; a transparentconductive oxide (TCO) electrically connected to the diffusion layer;and an overflow contact in electrical communication with the TCO.
 2. Thesystem as recited in claim 1, wherein the overflow contact is spacedapart laterally from the diffusion layer.
 3. A system as recited inclaim 1, wherein the pixel is one of a plurality of similar pixelsarranged in a grid pattern, wherein each pixel has a respective overflowcontact, forming an overflow contact grid offset from the grid pattern.4. The system as recited in claim 1, wherein the overflow contact ismetallic.
 5. The system as recited in claim 1, further comprising a caplayer deposited on the absorption layer opposite a substrate.
 6. Thesystem as recited in claim 5, wherein the cap layer includes InP.
 7. Thesystem as recited in claim 5, further comprising a SiNx layer over thecap layer.
 8. The system as recited in claim 7, wherein the TCO isdeposited on the SiNx layer, wherein the TCO conforms around the SiNxlayer to contact the diffusion layer.
 9. The system as recited in claim7, further comprising at least one additional SiNx layer deposited overthe TCO, wherein the overflow contact extends through the at least oneadditional SiNx layer.
 10. The system as recited in claim 5, furthercomprising an anti-reflective layer deposited on the substrate oppositethe absorption layer.
 11. The system as recited in claim 1, furthercomprising a contact metal electrically connected to the diffusionlayer, configured to electrically connect the diffusion layer to aread-out integrated circuit (ROIC).
 12. The system as recited in claim11, further comprising the ROIC connected in electrical communicationwith the contact metal.
 13. The system as recited in claim 12, whereinthe overflow contact is in electrical communication with the ROIC. 14.The system as recited in claim 13, wherein the overflow contact iselectrically isolated from the contact metal, and wherein the TCO iselectrically connected to a current Sink of the ROIC.
 15. The system asrecited in claim 13, wherein the TCO includes multiple layers of ZnO,TiO2 and/or Indium Tin Oxide (ITO).
 16. The system as recited in claim1, wherein the absorption layer includes InGaAs, and wherein the pixelis sensitive to illumination in infrared wavelengths.
 17. The system asrecited in claim 1, wherein the absorption layer includes Si, andwherein the pixel is sensitive to illumination in visible lightwavelengths.
 18. A method comprising: forming a pixel array including aplurality of pixels, each pixel including a diffusion layer in contactwith an absorption layer; and forming a respective overflow contactelectrically connected with each respective pixel of the plurality ofpixels, wherein the overflow contacts follow an overflow contact grid.19. The method as recited in claim 18, further comprising: depositing aSiNx layer on a cap layer that is deposited on the absorption layer; anddepositing a transparent conductive oxide (TCO) on the SiNx layer,wherein the overflow contacts are electrically connected to the TCO. 20.The method as recited in claim 19, further comprising depositing atleast one additional SiNx layer on the TCO.